Concrete casting of utility poles, e.g., poles used for supporting lighting fixtures and/or utility lines, is known. Such elongated structures have been cast in various cross-sectional shapes, e.g., circular, rectangular and octagonal. In a conventional process, concrete slurry is poured into a mold having the desired shape and is allowed to cure before removal of the mold from the casting (or removal of the casting from the mold). Typically, the mold contains reinforcement elements, e.g., rebar running longitudinally within the mold, that become part of the cast product and impart additional tensile strength to the cast concrete (which by itself has high compressive strength but very low tensile strength).
The assignee of the present application has, for more than a year, commercially produced rebar reinforced (non-prestressed) octagonal concrete lighting poles employing a clamshell-type mold form. In this process, the clamshell-type mold form is closed onto an elongated rail (i.e., pallet) supported on a pair of light-duty saw horse-like supports formed from lengths of angle iron. An octagonally shaped mold cavity is formed with the pallet top surface forming a lower surface of the resultant mold; the clamshell-type mold form forms six additional mold surfaces and an open top along which the eighth surface of the casting is formed. Concrete is poured and allowed to cure. Once the concrete has firmed-up, hinged halves of the clamshell-type mold form may be opened to permit lateral removal of the mold form from the casting. The mold form can be reused while the casting continues to cure on the pallet. Once curing is substantially complete, the casting may be removed from the pallet for finishing operations, storage, transportation, etc.
Cast concrete structures with substantially increased tensile strength can be obtained through known concrete prestressing techniques. Generally, in such known techniques, concrete is poured around high strength steel wires, cables or rods which are kept under considerable tension until the concrete has substantially completely set. The wires are then cut, and compressive forces are thereby imparted to the concrete through the bond between the steel and concrete. Additional tensile strength in the cast product results from the fact that when the structure receives a load, the compression imparted to the concrete by the prestressing elements is relieved on that portion that would otherwise be put into in tension by the load. In order to assure a strong bond between the tensioned steel wires and the concrete (which is required to avoid slippage), it is necessary to permit the casting to substantially completely cure before the tensioning elements are cut or otherwise disconnected from the tensioning fixture.
For certain high load applications, such as utility and lighting poles to be used in regions susceptible to high winds, e.g., hurricanes, the substantially greater structural strength afforded by prestressed concrete is highly desirable. As compared with conventional reinforced concrete casting operations, however, known industry techniques for casting prestressed concrete poles are labor and time intensive, and require additional materials (e.g., stressing elements) and costly casting (and tensioning) apparatus.
Pour casting into an open-top mold incorporating prestressing elements has been used to form prestressed concrete structures. As noted above, however, in such processes it has been necessary for the casting to remain in the mold form until the casting is substantially cured, in order to avoid slippage of the tensioned prestressing elements within the concrete. This may take between 16 and 20 hours. With the mold form occupied for this lengthy period of time, production rates per mold form are necessarily very low. To achieve higher production rates, it is necessary to employ additional mold forms (and associated tensioning apparatus), at concomitantly greater expense. Additionally, with apparatus known in the industry, castings having a cross-sectional dimension that reaches a maximum between opposed sides of the casting, e.g., octagonal poles, cannot easily be removed laterally from a mold cavity. Rather, removal of this type of casting from its mold cavity requires either an involved disassembly of the mold form, or an endwise removal operation, i.e., a longitudinal extraction of the casting from the mold form. In order to permit an endwise removal operation, an end wall of the mold form must be disassembled and removed. If the ends of the mold form are reinforced and specially configured to serve also as pretensioning headers, such a removal operation can to be difficult.
Centrifugal (spin) casting can be carried out to cast poles within a mold including tensioned prestressing elements. Such apparatus tend to be very costly, however.
The following patents teach particular apparatus and methods for casting elongated prestressed concrete products utilizing a mold form positioned between, or incorporating therein, headers of a wire pretensioning fixture:
COLLIER U.S. Pat. No. 832,594
DEIGAARD U.S. Pat. No. 3,269,494
CAZENAVE et al. U.S. Pat. No. 4,758,393
COLLIER discloses a mold forming a pair of opposed cavities of L-shaped cross-section, along which wires pretensioned between end brackets extend. Once the casting is set, the tensioned wires are severed at their ends, and the castings (angle posts) are removed from the mold box.
DEIGAARD discloses a mold form of rectangular cross-section intended for casting concrete poles having a longitudinal opening therethrough. The mold form is positioned within a wire pretensioning apparatus including an elongated very heavy base, which is preferably made of concrete and partially embedded in the ground. Pretensioning wires are extended between a pair of headers. The mold form itself is a multi-part open top structure. Once the concrete has hardened to such a point that engagement of the concrete with the tensioned cables will prevent any movement of the cables within the concrete, the cables are cut and the side plates of the mold are dismantled and removed.
CAZENAVE et al. disclose a mold form (“impression”) separable from a wire tensioning frame for use in making beams of prestressed concrete. Separability of the impression from the tensioning frame permits removal of the impression from the casting (and tensioning frame) for reuse in casting another beam while the first beam begins to dry. In the disclosed process, the tensioning frame is placed on the impression. Concrete is cast into the mold constituted by the combination of the frame and the impression. The concrete and the mold is vibrated and “rammed.” Then, the impression is flipped over together with the tensioning frame, and the impression is removed from the partially cured casting for reuse. The mold form nests within the tensioning frame; the frame comprises sides and ends but no central floor.